1. Technical Field of the Invention
This invention relates generally to mixed signal integrated circuits and more particularly to powering a real time clock module and/or crystal oscillation circuit of a system on a chip.
2. Description of Related Art
In general, a system on a chip (SOC) integrates multiple independent circuits, which are typically available as individual integrated circuits, on to a single integrated circuit. For example, an audio processing SOC combines a processing core (e.g., microprocessor and/or digital signal processor, instruction cache, and data cache), an audio codec (e.g., digitization of analog audio input signals and converting digitized audio signals into analog output signals), a high speed serial interface (e.g., universal serial bus (USB) interface), a real time clock (RTC), a crystal oscillation circuit, and an external memory interface.
When an audio processing SOC is incorporated in a battery powered device, the RTC and the crystal oscillation circuit need to be powered directly from the battery and not from a power supply (e.g., DC-DC converter) that is enabled when the battery powered device is enabled. One issue with powering the RTC and the crystal oscillation circuit from the battery is the use of different batteries (e.g., an alkaline battery produces a voltage of 0.9 to 1.5 volts and a lithium-ion battery produces a voltage of 3.0 to 4.2 volts). One known solution is to provide a linear regulator that generates a supply voltage of approximately equal to the battery voltage when an alkaline battery is used and generates a supply voltage of approximately one-half the battery voltage when the lithium-ion battery is used.
While this technique works for integrated circuit (IC) fabrication processes that have power supply requirements of up to 2.0 volts, newer IC fabrication processes (e.g., 90 nano-meter CMOS) have much lower power supply voltage limitations (e.g., less than 1.3 volts). As such, for SOCs developed using new IC fabrication processes, a new technique is required to power the RTC and the crystal oscillation circuit from the battery.
Another issue with the RTC is the potential loss of data at power down of the battery power device. As is known, the RTC includes a digital section and an analog section, where the analog section is powered from the battery and the digital section is powered from the DC-DC converter. At power down, the DC-DC converter is turned off, but as long as the battery is connected, the analog section of the RTC is active. When this state occurs and data is being transferred between the analog section and the digital section, the data can be corrupted or lost. The potential corruption or loss of data is even greater when the power down occurs near an edge of a clock signal.
Therefore, a need exists for an improved RTC module and an improved power generation technique for the RTC module and a crystal oscillation module.